The invention relates to a viscous coupling having and a hub are arranged to be rotatable relative to one another around an axis of rotation, and which form a closed unit. A set of inner plates are rotationally fast with the hub; having a set of outer plates which are attached in the housing in a rotationally fast way. The plates of at least one set of plates are movable in the direction of the axis of rotation. The inner plates and the outer plates are alternately arranged in the direction of the axis of rotation in a certain sequence while at least partially overlapping one another. The inner plates and/or the outer plates comprise apertures. A viscous medium at least partially fills the remaining spaces between the housing, the hub and the plates.
Viscous couplings are used in the drivelines of motor vehicles. For example, if slip occurs at the front wheels of a front wheel drive vehicle, the rear wheels can be connected by means of a viscous coupling in that the rear wheels are driven by a second gearbox output, a viscous coupling, a propeller shaft, a differential and sideshafts.
From DE-OS 21 35 791 it is known to provide the plates associated with one set of plates with apertures in the form of bores. The plates of the other set comprise slots. By designing the plates in such a way, it is possible to improve the torque capacity of such couplings. Furthermore, the degree of heat dissipation is increased by improving the circulation of the viscous medium.
At high viscous coupling speeds such as may occur if the viscous coupling is used in the driveline of a motor vehicle and if the vehicle travels at a high speed, centrifugal forces acting on the viscous medium separate the viscous medium and the air if, as usual, the viscous coupling is only partially filled with the viscous medium. In consequence, the inner regions of the plates, i.e. those regions positioned close to the axis of rotation, are either not moistened by the viscous medium or they are moistened to a lesser extent than the outer regions. As a result, wear symptoms occur at the inner regions at an earlier stage.
DE 37 02 299 C1 describes a viscous coupling wherein the inner and outer plates are provided with slots, with the slots of the inner plates and those of the outer plates intersecting one another. As a result, in the regions of intersection, there are formed cavities which extend from one plate to the next and which, in the case of a relative rotational movement of the plates relative to one another, move radially inwardly or outwardly, depending on the direction of rotation. This means that viscous medium is transported inwardly or outwardly, the purpose being to provide the coupling with a torque capacity which is dependent on the direction of rotation. However, when the viscous medium is transported outwardly, the separation of viscous medium and air at high coupling speeds is assisted, as a result of which plate wear is intensified.